Ocean thermal energy conversion (“OTEC”) is a method for generating electricity based on the temperature difference that exists between deep and shallow waters of a large body of water, such as an ocean, sea, gulf, or large, deep lake. An OTEC system utilizes a heat engine (i.e., a thermodynamic device or system that generates electricity based on a temperature differential) that is thermally coupled between relatively warmer shallow and relatively colder deep water.
One heat engine suitable for OTEC is based on the Rankine cycle, which uses a low-pressure turbine. A closed-loop conduit containing a fluid characterized by a low boiling point, such as ammonia, is thermally coupled with warm water at a first heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor is forced through the turbine, which drives a turbo-generator. After exiting the turbine, the vaporized working fluid is condensed back into a liquid state at a second heat exchanger where the closed-loop conduit is thermally coupled with cold water. The condensed working fluid is then recycled through the system.
OTEC systems have been shown to be technically viable, but the high capital cost of these systems has thwarted commercialization. The heat exchangers are the second largest contributor to OTEC plant capital cost (the largest is the cost of the offshore moored vessel or platform). The optimization of the enormous heat exchangers that are required for an OTEC plant is therefore of great importance and can have a major impact on the economic viability of OTEC technology.
Many types of heat exchangers have been employed in OTEC systems including; plate-fin, plate-frame, and shell-and-tube heat exchangers. Shell-and-tube heat exchangers are particularly attractive for use in OTEC applications because of their potential for large volume fluid flow and low back pressure. A shell-and-tube heat exchanger comprises multiple tubes placed between two tube sheets and encapsulated in a pressure-vessel shell. A first fluid or gas is passed through the tubes and a second fluid or gas is passed through the pressure-vessel shell such that it flows along the outer surface of the tubes. Heat energy is transferred between the first fluid and second fluid through the walls of the tubes. The tube ends are typically press fit or welded into the tube sheets.
Unfortunately, shell-and-tube heat exchangers have several drawbacks that have thus-far limited their use in marine applications. First, the overall heat transfer coefficient, U, that is associated with reasonable pressure drops for OTEC is typically below 2000 W/m2K. Heat transfer efficiency is limited by, among other things (1) baffles that are typically included in the pressure-vessel to induce turbulence and transverse flow of the second fluid, and (2) a limitation on the flow rate of the second fluid to avoid inducing vibration and flow forces that induce mechanical stresses and strains on the tubes.
A second drawback of conventional shell-and-tube heat exchangers is that they are prone to “bio-fouling.” Bio-fouling decreases efficiency and leads to increased maintenance costs (particularly for heat exchangers located at deep-water levels). Bio-fouling arises from, among other things, trapping of organic matter in voids and crevices, such as those associated with tubes that are press fit or fusion-welded into tube sheets.
A third drawback of conventional shell-and-tube heat exchangers is that they are not well-suited to seawater applications, such as OTEC. Since the tubes are press fit or fusion-welded into the tube sheets, it is difficult to ensure fluidic isolation between the primary fluid inside the tubes and seawater flowing around the tubes through the shell. Further, the reliability of conventional shell-and-tube heat exchangers is compromised by galvanic corrosion that occurs at the welded joints used to seal the tubes to the tube sheets. Galvanic corrosion occurs due to reactivity between dissimilar metals included in fusion welds. Galvanic corrosion is exacerbated by exposure of the welds to seawater. Reliability is degraded further by the potential for crevice corrosion in regions of stagnant flow even for shell-and-tube designs customized for OTEC applications.
Historically, these drawbacks have driven the size and cost for shell-and-tube heat exchangers too high for practical consideration.
With today's growing need for energy, using a renewable, constant, baseload power source is a desirable solution. As a consequence, there is a renewed interest in OTEC power plants. But development of a low-cost OTEC heat exchanger having high heat-exchange capacity, high flow rates, low pumping parasitic losses, and long life in the ocean environment remains elusive.